Integrated Photo Monitor Circuit and Optical Pickup Unit

ABSTRACT

The invention provides an integrated photo monitor circuit ( 12 ), particularly an integrated photo monitor circuit ( 12 ) for CD and DVD applications, comprising a photo detector ( 14 ), an amplifier ( 16 ) for amplifying an output signal of said photo detector ( 14 ), and sampling circuitry ( 18 ) for sampling an output signal of said amplifier ( 16 ). The invention further provides an optical pickup unit ( 10 ), particularly an optical pickup unit ( 10 ) for CD and DVD applications, comprising means ( 20, 22 ) for emitting light, and means ( 12 ) for generating a sampled feedback signal ( 40 ) correlated to the output power of said means ( 20, 22 ) for emitting light and intended to be evaluated for controlling said output power of said means ( 20, 22 ) for emitting light. By providing the sampling function on the optical pickup unit ( 10 ), especially in an integrated photo monitoring circuit ( 12 ), the necessary bandwidth of a flex connection ( 32 ) between the optical pickup unit ( 10 ) and a printed circuit board ( 42 ) may be reduced compared to solutions where the sampling is done on the printed circuit board ( 42 ).

FIELD OF THE INVENTION

The present invention relates to integrated photo monitor circuits formonitoring the output power of light sources like laser diodes. Theinvention also relates to optical pickup units, for example used inDVD/CD recording and playback devices.

BACKGROUND OF THE INVENTION

For example known CD/DVD read and write devices comprise an opticalpickup unit for reading and writing data on optical data carriers. Theoptical pickup unit comprises at least one light emitting device like alaser diode for emitting light onto the optical data carrier and meansfor receiving light reflected from the optical carrier. It is well knownto control the output power of the laser diode to achieve proper readingand writing results.

For controlling the output power of the laser diode it is necessary todetermine the actual output power of the laser diode. Therefore, knownoptical pickup units comprise a photo detector, usually a photo diode,detecting light directly emitted from the laser diode and/or reflectedfrom the optical data carrier. The output signal of the photo detectoris amplified, usually by a current to voltage amplifier, and thensampled and evaluated for controlling the output power of the laserdiode. With known solutions the output signal of the photo detector isamplified on the optical pickup unit and the sampling and furtherevaluating is done on a printed circuit board connected to the opticalpickup unit via a flex connection. The printed circuit board comprisescircuitry creating a laser power control signal depending on the actuallaser power, wherein the laser power control signal in turn istransported back to the optical pickup unit via the flex connection.

The advantage of using an integrated photo monitor circuit, for examplerealized in BiCMOS technology, instead of a discrete photo diode incombination with a discrete current to voltage amplifier to providelaser power feedback information is well known in the art. Integratedphoto monitor circuits where the photo detector and the current tovoltage amplifier are integrated have a substantially higher bandwidthcompared to solutions comprising a discrete photo diode and a discretecurrent to voltage amplifier.

To perform a proper sampling, it is additionally necessary in many casesto further supply timing information to the sampling circuitry. Suchtiming information may for example be delivered by means located on theoptical pickup unit for creating the write strategy. In this case alsothe timing information has to be supplied to the printed circuit boardvia the flex connection.

With the prior art there is a problem in that it is expensive to providea flex connection between the optical pickup unit and the printedcircuit board comprising the necessary bandwidth for transporting thehigh frequency laser feedback and timing information, respectively.

It is therefore the object of the present invention to further developthe known integrated photo monitor circuits and the known optical pickupunits such that the necessary bandwidth of the flex connection betweenthe optical pickup unit and the printed circuit board is reduced.

SUMMARY OF THE INVENTION

The above object is solved by the features of the independent claims.Further developments and preferred embodiments of the invention areoutlined in the dependent claims.

In accordance with a first aspect of the present invention, there isprovided an integrated photo monitor circuit, particularly an integratedphoto monitor circuit for CD and DVD applications, comprising a photodetector, an amplifier for amplifying an output signal of said photodetector, and sampling circuitry for sampling an output signal of saidamplifier. Such an integrated photo monitor circuit is able to deliver alaser power feedback signal requiring only a relatively small bandwidthfor transmission. Therefore, if the integrated photo monitor circuit inaccordance with the invention is located on an optical pickup unit it isno longer necessary to transport the analog forward laser power feedbacksignal and the timing information, respectively, over the flexconnection between the optical pickup unit and the printed circuitboard. Instead, only a sampled analog output signal representing theactual laser output power has to be transported over the flexconnection. This reduces the necessary bandwidth of the flex connection.

The integrated photo monitor circuit in accordance with the presentinvention preferably further comprises an input for receiving timinginformation used for sampling. Such timing information may for examplebe created by a laser driver which is also intended to create the writestrategy.

Furthermore, it is preferred to fabricate the integrated photo monitorcircuit in accordance with the present invention in CMOS or BiCMOStechnology. Especially the BiCMOS technology has an excellentprice-performance ratio.

It is also preferred that said amplifier of the integrated photo monitorcircuit is a current to voltage amplifier. The resulting voltage signalcan directly be used for sampling, in many cases.

In this context it is furthermore advantageous if the integrated photomonitor circuit in accordance with the present invention is adapted tobe mounted to an optical pickup unit. For example, the integrated photomonitor circuit may comprise contacts suitable for any type of knownsoldering processes.

In accordance with a second aspect of the present invention, there isprovided an optical pickup unit, particularly an optical pickup unit forCD and DVD applications, comprising means for emitting light, and meansfor generating a sampled feedback signal correlated to the output powerof said means for emitting light and intended to be evaluated forcontrolling said output power of said light emitting device. Thereby, itis also no longer necessary to transport the analog forward laser powerfeedback signal and the timing information over the flex connectionbetween the optical pickup unit and the printed circuit board. Even inthis case, only a sampled analog output signal representing the actuallaser output power has to be transported over the flex connection, andthe necessary bandwidth of the flex connection is reduced.

In preferred embodiments of the present invention, the optical pickupunit further comprises an interface for connecting said optical pickupunit via a flex connection to a printed circuit board comprising acontroller for controlling said output power of said means for emittinglight. The interface preferably enables a high frequency data transportbetween the printed circuit board and the optical pickup unit and allowsthe controller located on the printed circuit board to supply a laseroutput power control signal back to the optical pickup unit.

It may also be advantageous if the optical pickup unit comprises acontroller for controlling said output power of said means for emittinglight. In this case the bandwidth needed for communication between theoptical pickup unit and the printed circuit board is further reducedsince, for providing the laser output power control signal no bandwidthof the flex connection is required.

Furthermore, it is preferred in accordance with the present inventionthat said means for generating said sampled feedback signal furthercomprise a photo detector detecting light emitted by said means foremitting light. This photo detector may for example be realized by atleast one photo diode and, if necessary, by further circuitry.

Depending on the respective embodiment of the optical pickup unit inaccordance with the present invention, said means for generating asampled feedback signal further comprise a current to voltage amplifierfor amplifying an output signal of said photo detector, and samplingcircuitry for sampling an output signal of said current to voltageamplifier.

Without being limited thereto, it is preferred for all embodiments ofthe invention that said means for generating said sampled feedbacksignals are realized by an integrated photo monitor circuit,particularly by an integrated BiCMOS photo monitor circuit. The use ofsuch an integrated photo monitor circuit enables a reduction of thespace required on the optical pickup unit for providing the means forgenerating the sampled feedback signals, a reduction of the overallweight of the optical pickup unit, and provides a high bandwidth.Employing the BiCMOS technology for fabricating the means for generatingsampled feedback signals ensures fast information processing, low powerconsumption, and an excellent price-performance ratio.

It is particularly advantageous if said means for generating saidsampled feedback signal receive timing information generated on saidoptical pickup unit by means for creating a write strategy for writingto an optical data carrier, wherein said timing information is used forsampling. With the write strategy generator on the optical pickup unit,the timing information needed to sample the forward sense signal, i.e.the signal proportional to the actual laser power, is easily generated.

Furthermore, it is preferred that said means for emitting light comprisea laser diode. By the application of a laser diode excellent beamqualities, high output power and high data rates can be achieved.

All devices for reading and/or writing optical storage media comprisingan integrated photo monitor circuit in accordance with the presentinvention and/or an optical pickup unit in accordance with the presentinvention fall within the scope of the accompanying claims.

It is the gist of the present invention to reduce the high frequencybandwidth needed for the communication between an optical pickup unitand a printed circuit board by performing the sampling of a forwardlaser output power feedback signal on the optical pickup unit instead ofon the printed circuit board. This is very efficiently possible byproviding an integrated photo monitor circuit in accordance with theinvention on an optical pickup unit in accordance with the invention.Although the invention may advantageously be used in the field ofreading/writing optical data carriers, especially the integrated photomonitor circuit in accordance with the invention may also be used inother technical fields where high optical data rates have to beprocessed, for example in optical communication components or rangefinding applications.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified block diagram of an optical pickup unit inaccordance with an embodiment of the present invention comprising anintegrated photo monitor circuit in accordance with an embodiment of thepresent invention, as well as a printed circuit board connected to theoptical pickup unit by a flex connection.

DESCRIPTION OF PREFERRED EMBODIMENTS

It is to be noted that FIG. 1 only shows elements necessary forunderstanding the present invention, and that practical embodiments ofoptical pickup units may comprise additional elements not shown inFIG. 1. Furthermore, in practical embodiments the blocks shown in FIG. 1may be further divided or combined in any suitably manner. The arrowsshown in FIG. 1 represent signals which, in practical embodiments, maybe supplied by one or more lines, as appropriate.

In FIG. 1 there is shown an optical pickup unit (OPU) 10 connected to aprinted circuit board (BCP) 42 via a flex connection 32. The opticalpick up unit 10 is intended to write data to and read data from anoptical data carrier 52 which, in the embodiment shown, may be a CD orDVD. The optical pick up unit 10 comprises a laser driver 24 for drivinga first laser diode (CD LD) 20 adapted to operate at CD wavelengths anda second laser diode (DVD LD) 22 adapted to operate at DVD wavelengths.Furthermore, the optical pickup unit 10 comprises an integrated photodetector circuit (PDIC) 28 which for the purpose of reading data fromthe optical data carrier 52 is adapted to receive light emitted by thelaser diodes 20 and 22 and reflected by the optical data carrier 52.However, the integrated photo detector circuit 28 is not relevant forunderstanding the present invention. On the optical pick up unit 10there is further provided an integrated photo monitor circuit (PMIC) 12comprising a photo detector portion 14, a current to voltage amplifierportion 16 and a sampling portion 18. The optical pick up unit 10 alsocomprises an interface 30 adapted to couple the optical pick up unit 10to the printed circuit board 42 via the flex connection 32. A similarinterface 46 is provided for the same purpose on a printed circuit board42, which additionally comprises a laser power controller 30. Inpractical embodiments the printed circuit board 42 comprises a completechip set (not shown) for performing various data processing functionsnecessary in CD and DVD applications.

In operation, one of the laser diodes 20, 22 emits light with an outputpower which is controlled by a closed loop. For providing this closedloop, light directly emitted by one of the laser diodes 20, 22 and/orlight reflected from the optical data carrier 52 is received by thephoto detector portion 14 of the integrated photo monitor circuit 12.The photo detector portion 14 outputs a signal which is amplified by thecurrent to voltage amplifier portion 16 of the integrated photo monitorcircuit 12. In accordance with the present invention, the integratedphoto monitor circuit 12 comprises a sampling portion 18 for samplingthe amplified output signal of the current to voltage amplifier 16. Toachieve a proper sampling result, it is necessary to provide timinginformation to the sampling portion 18. In the present case, this timinginformation is created by a write strategy generator 26 which is part ofthe laser driver 24. The timing information is supplied by a timingsignal 36 to the integrated photo monitor circuit 12 and the samplingportion 18, respectively. The sampling portion 18 outputs a sampledlaser power forward feedback signal 40 which is supplied to the laserpower controller 30 via the interface 30, the flex connection 32 and theinterface 46. The laser power controller 30 evaluates the sampled laserpower forward feedback signal 40 and creates a laser power controlsignal 38 which is fed to the laser driver 24 via the interface 46, theflex connection 32 and the interface 30. The laser driver 24 drives thelaser diode 20 or 22 depending on the laser power control signal 38, andthereby the laser power control loop is closed.

All means mentioned herein may be realized by any suitable analog ordiscrete components well known to the person skilled in the art. Alsothe use of any type of signal processor is intended to fall within thescope of the accompanying claims. As regards the integrated photomonitor circuit 12, it is preferred that this integrated circuit isrealized, at least in part, in BiCMOS technology, as already mentioned.

Furthermore, equivalents and modifications not described above may alsobe employed without departing from the scope of the invention, which isdefined in the accompanying claims.

1. An integrated photo monitor circuit (12), particularly an integratedphoto monitor circuit (12) for CD and DVD applications, comprising: aphoto detector (14), an amplifier (16) for amplifying an output signalof said photo detector (14), and sampling circuitry (18) for sampling anoutput signal of said amplifier (16).
 2. The integrated photo monitorcircuit (12) according to claim 1, further comprising an input forreceiving timing information (36) used for sampling.
 3. The integratedphoto monitor circuit (12) according to claim 1, wherein said integratedphoto monitor circuit (12) is an integrated BiCMOS or CMOS circuit. 4.The integrated photo monitor circuit (12) according to claim 1, whereinsaid amplifier (16) is a current to voltage amplifier.
 5. The integratedphoto monitor circuit (12) according to claim 1, which is adapted to bemounted to an optical pickup unit (10).
 6. An optical pickup unit (10),particularly an optical pickup unit (10) for CD and DVD applications,comprising: means (20, 22) for emitting light, and means (12) forgenerating a sampled feedback signal (40) correlated to the output powerof said means (20, 22) for emitting light and intended to be evaluatedfor controlling said output power of said means (20, 22) for emittinglight.
 7. The optical pickup unit (10) according to claim 6, furthercomprising an interface (30) for connecting said optical pickup unit(10) via a flex connection (32) to a printed circuit board (42)comprising a controller (30) for controlling said output power of saidmeans (20, 22) for emitting light.
 8. The optical pickup unit (10)according to claim 6, further comprising a controller for controllingsaid output power of said means (20, 22) for emitting light.
 9. Theoptical pickup unit (10) according to claim 6, wherein said means (12)for generating said sampled feedback signal (40) further comprise aphoto detector (14) detecting light emitted by said means (20, 22) foremitting light.
 10. The optical pickup unit (10) according to claim 9,wherein said means (12) for generating a sampled feedback signal (40)further comprise: a current to voltage amplifier (16) for amplifying anoutput signal of said photo detector (14); and sampling circuitry (18)for sampling an output signal of said current to voltage amplifier (16).11. The optical pickup unit (10) according to claim 9, wherein saidmeans (12) for generating said sampled feedback signal (40) are realizedby an integrated photo monitor circuit (12), particularly by anintegrated BiCMOS photo monitor circuit (12).
 12. The optical pickupunit (10) according to claim 6, wherein said means (12) for generatingsaid sampled feedback signal (40) receive timing information (36)generated on said optical pickup unit (10) by means of creating a writestrategy for writing an optical data carrier (52), wherein said timinginformation (36) is used for sampling.
 13. The optical pickup unit (10)according to claim 6, wherein said means (20, 22) for emitting lightcomprise a laser diode (20, 22).
 14. A device for reading and/or writingoptical storage media (52), characterized in that it comprises anintegrated photo monitor circuit (12) according to claim
 1. 15. A devicefor reading and/or writing optical storage media (52), characterized inthat it comprises an optical pickup unit (10) according to claim 6.